In recent years large screen televisions (e.g., having a horizontal screen dimension greater than 37 inches) have become commonplace in many consumer's homes. This is in large part due to the emergence of new types of display systems which have made cathode ray tube (CRT) televisions obsolete in the large screen television market. One such type of display system is a projection display system (e.g., rear projection (RP) televisions, front projection projectors), wherein a projection lens is used to project an image onto a screen. Television sets and projectors have been developed that use projection lenses to support large projected image sizes at a reasonable cost. Projection lenses may be used for either front or rear projection, depending on whether the lens is on the viewer side of the screen or behind the screen. Often front projection lenses are used in projectors while rear projection lenses are used in televisions.
In a front projection system, the projector and viewer are on the same side of the display surface, with the image from the projector reflecting from the display surface to the viewer. An optical system utilizes light engine and projection optics to project an image directly on a display surface. It is desirable in such systems to have a short throw distance. The throw distance of a projection system is given by the projection distance d divided by the diagonal length D of a display surface, wherein the diagonal D is measured from the opposite corners of the display surface. Projectors with a short throw distance can provide large images for projectors placed at close distances from a projection screen (e.g., in small home theatre rooms, classrooms, small meeting rooms, etc.).
A typical projector system can utilize short-throw, wide-angle lenses with an on-axis optical path. This has an advantage of limiting depth reductions, but does so at the cost of a more complex design (e.g., even though keystone distortion is not present, this approach still requires optical elements that are challenging to design and manufacture). The optical and geometric constraints manifest themselves as increased pincushion or barrel distortion and keystone distortion. The design of prior art systems has largely been constrained by the requirement of minimizing these distortions along with achieving a required Modulation Transfer Function (MTF), correcting for lateral color, and meeting lens F-number specifications, while satisfying cost-performance tradeoffs.
Prior art rear projection systems use screen assemblies that have low reflectance to light impinging on them from the front (by use of light absorbing materials) in order to provide a high contrast ratio. These screen assemblies also have a high transmittance for light impinging on them from the rear (by use of lenticular arrays and collimation of light) in order to provide high brightness. Light is typically collimated by using a Fresnel lens as part of the screen assembly. A Fresnel lens is a symmetrical circular structure (its optical center is located at the physical center, or on the axis of the projection light path) for on-axis projection systems. A Fresnel lens of a given focal length substitutes for a large circular plano convex lens of the same focal length. The diameter of such a Fresnel lens is at most the length of the display diagonal. These Fresnel lenses are typically thin, very flexible and expand with interior temperature rise. The image quality for on-axis projection systems is not very sensitive to variations in the central portion (around the optical axis) of a Fresnel lens' surface profile. Unfortunately, when these symmetrical Fresnel lenses are used in off-axis RP systems (e.g., the off axis projection lenses use the off axis portion of an on axis Fresnel lens) they become problematic very quickly when the offset becomes high (e.g., angle of incident greater than 60°). In addition, the Fresnel lens in off-axis RP systems must be carefully designed because the light impinges on the rear of the screen at incident angles that vary from a minimum near the bottom of the screen of from 0±5 degrees to a maximum of up to 80±5 degrees near the top of the screen. Accordingly, the collimation must be performed with very high incident angles. Unfortunately, Fresnel lenses in short throw configurations are highly susceptible to image distortion due to screen flatness problems when the angle of incident becomes very steep.